Antagonism between hosts and parasites is ubiquitous. The strong, reciprocal natural selection that results from host-parasite interactions (coevolution) can have pervasive effects on the biology of both partners. Because coevolution with parasites affects many facets of human health and disease, a robust understanding of the coevolutionary process is a biomedical imperative. Host-parasite coevolution often plays out at the molecular level. Functionally dissecting host-parasite interactions therefore requires the capacity to genetically manipulate both partners, something that is not possible in human systems. To fill this gap, researchers must look elsewhere on the tree of life. Plants and their insect herbivores are the most common host-parasite systems in nature, providing experimentally tractable models to illuminate general features of coevolution. The proposed work will identify the genes and proteins that mediate coevolution between white butterflies (Pieridae: Pierinae) and their Brassicales host plants (including Arabidopsis thaliana). Previous work on this classic coevolutionary system has uncovered the genetic and functional bases of plant resistance to herbivorous insects, but a complementary understanding from the parasite?s perspective is lacking. Aim 1 will employ ancestral protein reconstruction and experimental biochemistry to identify the mutational events that enable novel coevolutionary interactions. Aim 2 will use CRISPR/Cas9 genome editing to functionally dissect the butterfly genes mediating coevolution with Arabidopsis plants, enabled by a recent GWAS that identified intriguing candidate loci. Finally, Aim 3 will use population genomic and comparative genomic methods to characterize how alternative modes of coevolution shape the fate of genetic variants that determine parasite success. This work will result in a functionally validated, experimentally tractable model of host-parasite coevolution. Together, these projects will contribute to postdoctoral training at the interface of evolutionary genomics, functional genetics, and experimental biochemistry. New skills and independence gained through this training will facilitate the transition to a research career in evolutionary genomics. The University of Chicago is an exceptional setting to pursue this work due to the University?s strength across the biological sciences, expansive resources, and the particular expertise of co-sponsors and local collaborators.